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Geometric Design Elements 75 Exhibit 5-76. Example design for a downhill situation. 16 ft vertical curve gutter 0% 30 @ 5. ft @ 15 ft 9.0 % sidewalk in flat roadway May need a part of curve vertical curve 45 ft @ When driveway slopes down 4.33% from the gutter: (1) water in gutter flows into With the alternate design: and down the driveway; (1) flow is confined to gutter, up (2) vehicles pulling into the to a depth of 0.47 ft; roadway must overcome the (2) vehicles entering the street grade. depart from a downhill platform. the gutter line. The 5-ft-wide sidewalk has a +2.0% cross slope, for a rise 0.1 foot, for total rise of 0.49 feet above the gutter line elevation. Exhibit 5-76 shows a design for a situation where the driveway would normally slope immedi- ately downward from the gutter line at a 4.33 percent grade. The alternate design (dashed line) allows the driveway to slope up from the gutter before sloping back down. Again, this design confines nor- mal depths of flow to the gutter, instead of allowing the gutter flow to rush down the driveway. Other Elements This section discusses other aspects of driveway design, such as landscaping, right-turn lanes on the roadway in advance of the driveway, surface drainage in the area where the driveway meets the roadway and sidewalk, use of traffic control device (e.g., signs, pavement markings, and traf- fic signals), and other situations. Landscaping and Business Signs Appropriate landscaping near roadway-driveway-sidewalk intersections can produce envi- ronmental and aesthetic benefits. Landscaping can also directly or indirectly help meet some geometric design objectives for one or more user groups. Landscaping can benefit driveway users in the following ways: Landscaping helps reduce stormwater run-off and soil erosion. Tree canopies can provide shade for pedestrians. Trees that shade pavement can reduce asphalt temperatures by as much as 36F and fuel tank temperatures by nearly 7F (5-27). Well-designed landscaping can help define driveway edges and make the driveway location more conspicuous. However, ill-chosen or ill-placed landscaping can be an inconvenience or even a hazard. Tree selection and suitability should consider climate, maintenance requirements, susceptibil- ity to disease, space available for root growth, ultimate tree height, and size of mature canopies. In more extreme cases, vegetation may physically interfere with one or more driveway user groups. Continuous maintenance of landscaping is essential to preserve plantings and sight lines, so the implications of budget limitations for maintenance should also affect landscaping decisions. Exhibit 5-77 presents suggested guidelines for the placement and control of vegetation (5-28).

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76 Guide for the Geometric Design of Driveways Exhibit 5-77. Landscaping guidelines for driveways. Concern or Issue Design Response Specific Procedure and/or Information Provide unobstructed Do not install landscaping that blocks Refer to the latest edition lines-of-sight among needed sight lines. of the AASHTO Green bicyclists, pedestrians, Trees should be set back a sufficient Book for the procedure and vehicles in the area distance from the driveway public road to calculate the needed intersections to avoid obstructing sight distance. lines. In urban settings, trees generally should be set back at least 20 to 30 ft on the approach to intersections and 10 to 20 ft on the far side. However, in higher speed environments, greater setbacks may be required (5-29). The top of ground cover in driveway and street medians should not exceed 2 feet. This is 18 in. below the clear sight line of 42 in. The bottom of the tree canopy should be at least 5 ft (60 in. high). Landscaping should not Vegetation should be sufficiently obscure or interfere with removed from traffic signs. traffic control devices or Vegetation should be sufficiently other roadside fixed removed from utility lines. objects Landscaping should not Limbs or branches that overhang any The ADA requires at least create conflicts in the pedestrian path should be at least 7 ft an 80 in. clearance paths of users above the surface of the path. above the pedestrian Vegetation should be sufficiently path (5-30). removed from pavement surfaces to prevent roots from damaging sidewalks. Vegetation should be sufficiently removed from pavement edges to avoid scraping vehicles. Planting arrangements should not create concealed spaces. Landscaping should not Trees should be set back at least 40 ft interfere with adequate from luminaries. artificial illumination Preserve an adequate Along major highways, the clear zone See AASHTO Roadside roadside clear zone should normally extend at least 10 ft Design Guide (5-31). beyond the edge of the shoulder. More study is needed to better define the needed clear zone in lower speed, built-up urban street environments. Business signs may be present outside of the roadway right-of-way, along driveways, or within parking areas. These signs should be placed so that they do not compete with traffic signs or obstruct sight lines of the various users. Along a busy roadway, a business sign may help identify a driveway location. If placed close to the driveway, a sign can help motorists who are scanning the upcoming roadside to detect the location of the driveway they are searching for. Conversely, a business sign located far from the driveway may actually divert a motorist's view from the driveway location and be misleading and confusing. Auxiliary Right-Turn Lanes Right-turn deceleration lanes are frequently constructed to remove the slower right-turning vehicles from the through travel lanes when right-turn volumes into a driveway are heavy and/or could have a significant adverse effect on through traffic. The benefits that accrue from having right-turn lanes include increased capacity, reduced speed differentials and brake applications, and reduced rear-end collisions.

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Geometric Design Elements 77 When a pedestrian is crossing a driveway, a right-turn auxiliary lane on the public highway allows a driver to wait without blocking a through traffic lane. A right-turn lane also removes turning vehicles from the through traffic lane, thus limiting interference with traffic progres- sion through a coordinated traffic signal system. Right-turn lanes may be desirable, but, where provided, should not be continuous, to avoid additional conflicts that would be introduced with both vehicular and bicycle traffic. Installation Guidelines The decision to provide an auxiliary right-turn deceleration lane on the roadway approach to a driveway intersection is usually made by the governing transportation agency. Although the driveway designer may not be in a position to make a decision as to whether a right-turn decel- eration lane should be installed, it is important for the designer to have some background information as to how a decision is made and on how provision of a right-turn deceleration lane may affect the driveway. Considerations in the decision making process generally include roadway volumes and speeds, driveway volumes, right-turn volumes, type of traffic control at the driveway intersection, and property availability. Some states have established application and design criteria for right-turn deceleration lanes for driveways and intersections on roadways under their jurisdiction, but the criteria vary widely from state to state (5-32)--two examples follow. Colorado DOT has warrants for right-turn decelerations based on roadway classification and posted speed (5-33). For example, on a roadway classified as an Expressway, Major Bypass (Category E-X), a projected peak-hour right-turn ingress turning volume greater than 10 vph would warrant a right-turn lane. For a Non-Rural Arterial (Category NR-A), a right-turn lane would be warranted for any access with a projected peak-hour right-turn ingress turning vol- ume greater than 50 vph; if the posted speed is greater than 40 mph, a right-turn deceleration lane would be warranted for any access with a projected peak-hour right-turn ingress turning volume greater than 25 vph. Florida DOT has guidelines based on posted speed and volume (5-9, p.60). For roadways with a posted speed of greater than 45 mph, 35 to 55 or more right turns per hour would war- rant a right-turn lane. For roadways with a posted speed of less than or equal to 45 mph, 80 to 125 or more right turns per hour would warrant a right-turn lane. The lower thresholds would be most appropriate on higher volume roadways or on two-lane roadways where lateral move- ment is restricted. The research in NCHRP Report 420 may be applied to assess the effects of right turns on curb lane operations (5-12). The installation of a right-turn deceleration lane has implications in terms of potential con- flicts with pedestrian movements. The objective of NCHRP Project 3-89, "Design Guidance for Channelized Right-Turn Lanes," is to develop design guidance for channelized right-turn lanes, based on balancing the needs of passenger cars, trucks, buses, pedestrians (including pedestrians with disabilities), and bicycles. Design Considerations An auxiliary lane for either right- or left-turn lanes should be at least 10 feet wide and, ideally, should equal that of the through lanes. If the lane has curbs, the curb face should be appropri- ately offset from the lane edge (5-1). As shown in Exhibit 5-78, the length of the auxiliary lanes for turning vehicles consists of three components: entering taper, deceleration length, and storage length (5-1). Ideally, the total length of the auxiliary lane should be the sum of the length for these three components; however,

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78 Guide for the Geometric Design of Driveways Exhibit 5-78. Parts of a deceleration lane. common practice is to accept a moderate amount of deceleration within the through lanes and to consider the taper length as part of the deceleration length. The following paragraphs summarize each component of the auxiliary lane length, based on information in the AASHTO A Policy on Geometric Design of Highways and Streets (5-1). Additional information is available in this AASHTO document. Taper. On high-speed rural roadways, a common practice has been to use a taper rate between 8:1 and 15:1 (longitudinal-to-transverse). In urban areas, some use a standard taper ranging in length from 50 to 100 feet. The following numbers provide an example from an agency that varies the taper length according to the posted speed: Posted speed (mph) 30 35 40 45 50 55 Straight line ratios 6:1 8:1 10.5:1 12.5:1 14.5:1 16.5:1 Source: New Mexico DOT, State Access Management Manual, Ch. 8, Sec. 18, p. 92, Sep. 2001 Some considerations favor shorter tapers over longer tapers at urban intersections, including driveways: 1. Shorter tapers appear to produce better "targets" for approaching drivers and to give more positive identification to an added auxiliary lane. 2. A longer taper may cause some drivers to incorrectly think that the deceleration lane is a through lane, especially when the taper is on a horizontal curve. 3. For the same total length of taper and deceleration, a shorter taper allows the storage length to be longer. This results in a longer length of full-width pavement for the auxiliary lane. The additional storage length helps to avoid turning traffic backing up in the through travel lanes and the slower speeds during peak periods would have a shorter taper needed. However, at higher vehicle speeds, this would involve deceleration in the through or turn lane. 4. During peak periods, when the queue length in the auxiliary lane is longer, speeds may decrease, which will make a shorter taper adequate. Deceleration Length. Provision for deceleration clear of the through traffic lanes is desirable on arterial roadways. Exhibit 5-79 lists turn lane deceleration distances from different sources. The braking distance component of stopping sight distance is included for comparison. On many urban facilities, an auxiliary turn lane is not long enough to accommodate the stor- age and all of the deceleration within its limits. Therefore, the initial part of the deceleration takes place in the through lanes, before the vehicle enters the auxiliary lane. In some higher volume and speed environments, significant deceleration in the through lanes may affect safety and operations adversely, so deceleration in the through lanes should be minimized. For steep upgrades, a shorter deceleration length may be acceptable. For significant down- grades, the deceleration distances need to be extended.

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Geometric Design Elements 79 Exhibit 5-79. Example deceleration lengths. Deceleration Length (ft) Design AASHTO AASHTO Fla Fla NM NM Wis Speed Brakinga Comfortableb (Urban)c (Rural)d Stope 15mphf d2g (mph) Value Value Values 30 86 170 -- -- 200 175 160 35 118 -- 145 -- 250 230 -- 40 154 275 155 -- 325 300 275 45 194 340 185 -- 400 370 -- 50 240 410 240 290 475 450 425 55 290 485 -- 350 550 525 -- -- indicates no value given for this speed a. braking distance (2004, p. 112) b. deceleration (2004, p. 714) c, d. assumes 10 mph speed difference (from Std. Index 301, rev. 2005). The FDOT Driveway Handbook (2005, p. 63) says "Right turn lane tapers and distances are identical to left turn lanes under stop conditions." e, f. New Mexico State Access Mgmt. Manual, Ch. 8, Sec. 18, p. 92, Sep. 2001. One condition is deceleration to 0 mph, the other is to 15 mph. g. Wisconsin Facilities Development Manual, 11-25-1, p. 1-3, May 2006. Distance d2 is the distance traveled while the driver maneuvers laterally and stops. The values allow a 10 mph speed difference when the turning vehicle clears the through lane. Storage Length for Left-Turn Lanes. At unsignalized driveway intersections, the storage length, may be based on the number of turning vehicles likely to arrive in an average 2-min period within the peak hour. Storage for at least two passenger vehicles should be provided. Where trucks represent more than 10 percent of the traffic, storage should be sufficient for at least one car and one truck. Storage Length for Right-Turn Lanes. At unsignalized driveways, if the turn lane does not stop or yield to other motor vehicles, and pedestrians seldom cross the driveway, no storage may be needed.If pedestriansoften cross the driveway, then storage for at least one vehicle may be desirable. Drainage of Surfaces Occupied by User Groups When there is deep standing or flowing water, the following undesirable scenarios can occur: A passing motor vehicle will splash nearby bicyclists, pedestrians, or persons waiting at a tran- sit stop. In the more extreme cases, it may adversely affect a driver's ability to control a vehicle. Bicyclists and pedestrians are forced to wade through the water. A good driveway design considers and accommodates the flow of water that results from sur- face runoff in a way that minimizes inconvenience to users. Surface runoff water should flow toward a gutter, an inlet, a flume, a ditch, or other suit- able destination and not stand and pond in the roadway-driveway-sidewalk intersection area. Although it may be impossible to totally eliminate runoff, depths can be minimized and flows directed away from pedestrian users, the most vulnerable of the user groups. To achieve suitable drainage and avoid creating problems, the designer should examine the amount of and direction of surface flows in and near the intersection of the driveway with the roadway and sidewalk. The designer should specify the elevations of the surfaces of the driveway, the sidewalk, and the border on the design sheets. Drainage grates in the driveway can help intercept the surface runoff. As Exhibit 5-80 shows, if installed, grates do need to be inspected and maintained periodically to avoid creating poten- tially hazardous situations.

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80 Guide for the Geometric Design of Driveways Exhibit 5-80. Damaged grates in driveway need repair. Designing to avoid directing roadway gutter flow into a driveway and onto private property was discussed in the Vertical Alignment section. Traffic Controls Signs, pavement markings, and traffic signals are called traffic control devices (TCDs). Strictly speaking, they are not geometric design elements, but TCDs may be used to complement a geo- metric design. Because of low volumes and speeds, TCDs are not needed on many driveways. Driveways with moderate to high traffic volumes are more likely to need some form of traffic control, such as signs and/or pavement markings. Where TCDs are installed, they should be consistent with the signs and markings that motorists and pedestrians are familiar with, the ones they see on the surrounding roadway sys- tem. The Manual on Uniform Traffic Control Devices (MUTCD) (5-34) for streets and highways sets forth the guidelines for the application of traffic signs, pavement markings, signals, and other TCDs. Sign Considerations Among the many situations that call for signs, the following are likely to be found at driveways and perhaps overlooked by some designers: Along an undivided roadway, when triangular islands (pork chops) are constructed in the driveway entry throat to prohibit one or both left turns, installation of No Left Turn (R3-2) sign(s) in conformance with the MUTCD is needed. If a driveway has a wide median, drivers may find that the R4-7 Keep Right (of the median nose) sign is helpful. Driveways intended for one-way operation should be accompanied by appropriate signs, so motorists will not proceed in the wrong direction. Refer to the MUTCD for information about the use of One-Way, turn prohibition, and Do Not Enter signs. If a driveway connects with a narrow roadway, motorists may find parked cars make turn- ing into or out of the driveway difficult or impossible. Some situations may call for parking

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Geometric Design Elements 81 Exhibit 5-81. Wider driveways without and with pavement markings. (a) (b) prohibitions in advance of and past the connection and on the other side of the roadway oppo- site the connection. Marking Considerations The MUTCD requires that pavement markings separating opposite directions of travel, such as a center line, be yellow. Markings separating the same direction of travel (e.g., lane lines) and the outer edge lines are white, as are stop lines, crosswalk markings, and directional turn arrows. Where driveways are wide enough to accommodate three or more lanes of traffic, pavement markings to delineate the intended lanes should be provided. Exhibit 5-81 shows two drive- ways wide enough for three lanes of traffic. On the driveway without the pavement markings, motorists are much more likely to position their vehicles so as to create problems and conflicts with other vehicles. Some driveways with multiple exit lanes are marked with slightly offset stop lines, as shown in Exhibit 5-82. This is done so that when both left-turning and right-turning vehicles are try- ing to exit the driveway at the same time, the left-turning vehicle does not block the needed line- of-sight of the right-turning driver. The right-turning vehicle is given preference because a safe right-turn maneuver requires only an adequate size gap from the left, while a safe left-turn maneu- ver requires adequate size gaps from both the left and the right. This offset also accommodates the path of a vehicle turning left from the roadway into the driveway. Channelizing devices, such as tubular markers, have been used to enhance delineation and to reinforce turn prohibitions. They are sometimes part of a driveway triangular island installation. The MUTCD provides detailed instructions for using channelization devices, including that, if used at night, they are to be retroreflective. Exhibit 5-83 shows tubular markers used to discour- age unwanted left turns. Signal Considerations Where high-volume driveways intersect public roadways, traffic signals may be necessary. Considerations are listed in Exhibit 5-84. Driveways sometimes essentially form the fourth leg of a signalized intersection. The current MUTCD does allow a driveway that forms the fourth approach or leg of an otherwise signalized intersection to be unsignalized. Before deciding to exercise this option, the designer should ascer- tain that volumes and speeds on the other three approaches as well as on the driveway are low enough so that vehicles from the unsignalized driveway can safely enter the intersection.

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82 Guide for the Geometric Design of Driveways Exhibit 5-82. Offset stop line Exhibit 5-83. Example of tubular markers to prohibit markings. a movement. roadway driveway sidewalk yellow white Railroad Grade Crossings Where there is a practical alternate route, it is desirable that a driveway not cross railroad tracks at grade. However, in some cases, the only access to a public road may be across a rail- road track. The guidance in the Railroad-Highway Grade Crossing Handbook (5-35) for design of rail- highway crossings also applies to the design of driveway-rail crossings. Exhibit 5-85 lists some design considerations for driveways crossing railroad tracks. Track maintenance can result in raising the track as new ballast is added to the track struc- ture. The Handbook cautions that "unless the highway profile is properly adjusted, this practice will result in a `humped' profile that may adversely affect the safety and operation of highway traffic over the railroad." The greatest risk of becoming hung up at railroad-highway grade crossings because of contact with the track or highway surface is posed by low-clearance, long- wheelbase vehicles. A similar problem can occur where the crossing is in a sag vertical curve. In this case, the front or rear overhangs on certain vehicles can strike or drag the pavement. When a road parallels a railroad and an intersecting driveway crosses the railroad, a rail- road grade crossing near the roadway intersection results. The Railroad-Highway Grade Exhibit 5-84. Traffic signal considerations. Traffic Signal Design Element Suggested Practice Minimum green time To accommodate pedestrians crossing either the driveway or the roadway, the designer can either establish a minimum green time that is adequate for crossing or provide pedestrian pushbuttons. Users with disabilities Many situations, especially in urban areas, call for features such as detectable warnings to accommodate pedestrians with disabilities. Actuation In most cases, want an actuated signal, so will not take away green from the through roadway unless there is actual driveway demand. Semi-actuated may be adequate. Progression If the driveway traffic signal is one of a series along the roadway, then time and coordinate the signal to minimize interference with progression along the through roadway. Semi-actuated traffic signals may help minimize interruptions to through traffic on the public road.